'FreeLeft p' is 'forall x . exists y . p x y'
In other words the caller is free to specify the left witness,
and then the right witness is an existential.
Note that the order of the type constructors is important for ensuring
that y is dependent on the x that is supplied.
This is why Stepped is needed, rather than writing the more obvious
'Sealed (Poly p)' which would notionally have the same quantification
of the type witnesses.

'FreeLeft p' is 'forall y . exists x . p x y'
In other words the caller is free to specify the right witness,
and then the left witness is an existential.
Note that the order of the type constructors is important for ensuring
that x is dependent on the y that is supplied.